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首页 > 解决方案 > 来自较早承诺的控制台日志出现在来自最新承诺的控制台日志之后

javascript - 来自较早承诺的控制台日志出现在来自最新承诺的控制台日志之后

问题描述

下面不是我的确切代码，但我只是想概述一下结构。当我运行代码时，我得到控制台日志的顺序是这样的：

  1. '完毕'
  2. json

我希望这会反过来，因为为了让 function2 完成（并发送“完成”解析值）， function3 必须首先完成。

我想了解为什么它不能那样工作。

function1().then(() => {
      return function2()
    ).then((message) => {
      console.log(message)
    })

    function function2() {
      return new Promise((resolve, reject) => {
        fetch(url, {
            method: 'get',
            body: null,
            headers: {
              "Content-Type": "application/json; charset=UTF-8",
              "Accept": "application/json; charset=UTF-8",

            },
          })
          .then(res => res.json())
          .then((json) => {
            return function3(json)
          })

        resolve('Done')
      })
    }

    function function3(json) {
      console.log(json)
    }

标签： javascriptpromisesequenceconsole.log

解决方案

resolve您甚至在完成之前就打电话fetch了。

如果您将其移至另一个，它将起作用then：

// ...
.then(res => res.json())
.then((json) => {
    return function3(json)
})
.then(() => {
    resolve('Done')
})

但事实上，整个new Promise事情甚至都没有必要，因为fetch已经返回了一个承诺！

// ...

function function2() {
    return fetch(url, { // It's important to actually *return* the promise here!
        method: 'get',
        body: null,
        headers: {
            "Content-Type": "application/json; charset=UTF-8",
            "Accept": "application/json; charset=UTF-8"
        }
    })
    .then(res => res.json())
    .then((json) => {
        return function3(json)
    })
    .then(() => {
        return 'Done'
    })
}

这可以使用async/进一步简化await：

// I moved this into a function because the global module-level code
// is not in an async context and hence can't use `await`.
async function main () {
  await function1()
  console.log(await function2())
}

async function function1 () {
  // I don't know what this does, you didn't include it in your code snippet
}

async function function2 () {
  const response = await fetch(url, {
    method: 'get',
    body: null,
    headers: {
      "Accept": "application/json; charset=UTF-8"
    }
  })
  
  const json = await response.json()
  await function3(json)

  return 'Done'
}

// At the moment this wouldn't have to be async because you don't do anything
// asynchronous inside, but you had `return function3(json)` in the original code,
// so I assume it is *going* to be async later.
async function function3 (json) {
  console.log(json)
}

// Don't forget to .catch any rejections here! Unhandled rejections are a pain!
main().catch(console.error)

（当我这样做时，我删除了Content-Type标题，因为它对GET请求没有任何意义。）

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